This invention relates generally to imaging and lithography for microfabrication processes, and more particularly relates to electron beam imaging and lithographic processes.
Electron beam lithography (EBL or E-beam lithography) has become a critical tool for microelectronic fabrication processing and for nanoscience research. A wide range of applications, including microelectronics, nanoelectronics, optoelectronics, biological and biochemical sensing and analysis, and photovoltaic technologies all rely on EBL for the production of state-of-the-art systems. EBL is providing a step toward the resolution needed for nano-scale implementation of such devices and systems.
Conventionally, a lithographic process employing an electron beam is carried out with an organic photoresist, such as poly(methylmethacrylate) (PMMA). In one example of such a process, a microelectronic structure, such as a microelectronic substrate or wafer, is scanned with an electron beam to map the features on the structure and provide reference points for subsequent electron beam patterning of a layer of resist provided on the structure. Such a layer of resist, e.g., PMMA or other resist material, is then spun over the wafer. The resist layer is then exposed to a beam of electrons that is controlled to “write” a prespecified pattern of electrons across the layer surface. This direct writing of a pattern by an E-beam eliminates the need for a lithographic mask. The resist regions that are exposed to the E-beam writing pattern are irradiatively damaged, and during subsequent chemical development, are selectively removed. The unexposed resist regions remain on the wafer. Thereafter, a selected fabrication process, such as etching, metal deposition and lift-off, or doping, can be conducted with the patterned resist in place on the wafer, and the patterned resist subsequently removed, typically by a chemical solvent.
For many applications, the features that exist on a surface to be patterned by such an EBL process include non-planar, three-dimensional structures and topology and fragile structures, such as nanowires or single walled carbon nanotubes (SWCNTs), as well as fragile nanometric films such as graphene. For all of these features and materials, it is found that processing with an organic E-beam photoresist can produce residues that can contaminate devices and sensitize materials to contaminants, and can contaminate subsequent processes. Indeed, electron transport studies of very clean as-grown SWCNTs have demonstrated that organic resist contamination can obscure the intrinsic electrical properties of SWCNTs. Standard cleaning procedures for organic residue removal, such as oxygen plasma processes or highly oxidizing chemical treatments, have the unfortunate consequence that organic nanostructures, such as SWCNT and graphene, can themselves also be completely removed. As a result, conventional E-beam photoresist materials are often incompatible with the materials and structures for which EBL is required.
Even further, it is often found that E-beam mapping of a surface topology prior to E-beam patterning of a photoresist layer can damage or destroy features and topology on the surface. For example, SEM imaging of a carbon nanotube can accelerate contamination of the nanotube, can impact the intrinsic electrical properties of the nanotube, and may directly damage the nanotube. Imaging of such structures with an atomic force microscope (AFM) has been employed as an alternative to SEM imaging, but AFM imaging is conventionally extremely slow as well as highly inconvenient to integrate with an E-beam-based lithographic process. As a result, many nanoscale structures and materials cannot be imaged and/or patterned with conventional electron beam processes without perturbation of their native characteristics.